Scheduling Algorithms for Modern Disk Drives

Scheduling Bruce

Algorithms

for

L.\Vorthington,

Department

Gregory

of Electrical

University

subsystem by

quests.

performance

dynamically

Via

can

ordering,

strongly

validated

be

dramatically

scheduling,

or

simulation,

im-

pending

of

complex

logic&

caches

synthetic

on

scheduling

workloads

and

effectiveness.

academia

and

the

workload

exact

tational

user

environments,

traces

captured

we arrive

at three

large

Using

both

six

(I)

Incorporating

scheduler

provides

response which

complex

times

always

highest

such

(3)

among

the

and

for

highest

exploit

with

important

been

for

ing

and

subsystems the

storage

acterized

must

growing

replaced

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components.

by intense

of pending

bursts

requests

for dynamically

A

portion

prised

of

blocks

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account

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the

the

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various

can

providing

reasonable

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past

have

been

position.

entire

with

disk

years,

proposed

times a

for

variety and

of

be

most

of our

date

appear,

and

notice

these

features

disk

drives. of logi-

by a high-level to

affect

any

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buffers

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published Most

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the

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comparison

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more

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workloads

well-validated

six

scheduling

traces.

dk.k

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work data

disk

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avenues

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some 3

the

for

our

results

future due

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origins using

Section

excluded

[Wort

how

4 describes

respectively.

suggests

in

and

Section

Section

validation

descriptions,

be found

drives

accuracy.

research.

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marizes

itations,

modern

scheduling

including

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into

accesses,

7 sumresearch.

to space

lim-

94].

a

while

requests.

in

2 describes

can

workloads

2

Modern

(on

a common

A disk

alboth

Data

mation

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prefetching

systems.

random

titie

and

of

lacking

mapping

speed-matching

large

While

previous

of the

The

publication

Small

experiments

methodology,

Permission to copy without fee all or part of this material is granted provided that the copies are not made or distributed for direct commercial advantage, the ACM copyright notice and the the

modern the

unavailable

to allow

to drive

dia.

of

in allow

to be hidden

the validity using

workloads

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scheduling

implemented

to

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features

requests.

overhead

individual

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disk.

Section

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disk

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25

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delays,

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minimize

3].

on

compu-

algorithms.

dom workloads,

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creating

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positioning relative

current

scheduler

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of request

mechanical

dependent

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of activity,

[MciYu86,

is responsible significant

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how

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to compen-

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managed

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to synthetic

cache.

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performance

knowledge Most

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expanded

to physical

making

red-world Disk

is dependent

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read

position-

provided

blocks

various

Introduction

sate

available

used

has

we investigate sig-

achieves

overall

performance

a prefetching

in

algorithms

reduce

merit the

status.

features

logic

scheduling

which

order,

seek-reducing

nal

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(C-LOOK), logical

that

the

provide

workloads

the

current

algorithms

interface,

2YO) decrease (2)

relative

requests,

differ-

into

caches

algorithm

inascending

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produce

recognize

1

scan

performance

delays

disk

improvements cyclical

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algorithms.

prefetching

requests

workloads.

they

seek-reducing

The

schedules

(less

and

and

On-board

conclusions:

information

a marginal

utilize

performance

sequentiality.

ing

for

effectively

nificant

the

only

mapping

Their

of disk

the

and

from main

industry.

resources,

cal data ent

Science

urnich.edu

several prefetching

Patt

48109-2122

configuration

mappings

N.

re-

we examine

to-physicaJ

Drives

Yale

and Computer

scheduling impact

Disk

Ganger,

Ann Arbor

Abstract Disk

R.

Engineering

of Michigan, worthing~eecs.

proved

Modern

is given

copying is by permission of the Association of Computing Machinery. To copy otherwise, or to republish, requires a fee and/or specific permission.

SiGMETRICS 94- 5/94 Santa Clara, CA. USA 0 1994 ACM 0-89791 -659-xKWO005..$3.5O

241

drive

ten

unit

of data

error

tracks

(one

from

of the

disk.

For

a set

coated

writ

holds (e. g.,

Drives

contains

axis)

are

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commonly

that

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in

storage

data).

surface)

example,

the

plus

rotating with

a sector,

innermost

me-

which

header/trailer

A cylinder

equidistant

platters

magnetic

on each surface.

is usually

of data

correction

sides

tracks

circular

512? bytes

each

of rapidly

on both

from cylinder

infor-

is a set of the

center

contains

the

innermost

media

track

location

tor.

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disk

arm

be positioned

to

together

that

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a specific

of

surface.

by

holds

any

a given

given

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cylinder,

cylinder. disk

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at any

its

heads

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arm

drives

heads

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and

sec-

that

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are ganged

corresponds

one

use of memory

has

progressed

age.

head

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disk

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(seek)

to the

by

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delays. target

cylinder.

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head.

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spans

multiple

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disk

target

delays

suffer

arm

one

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rotational

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result

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move

to

data

the

reach

on-board The

be-

Current affect

desired

data

are

the

drives

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accuracy

result

of of

self-managed. pacity goal

teristics

work

should

to

make

also

factor

into

disks

have

determine

that

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algorithms.

in

a disk

a major

exactly

scheduling

drive’s

ca-

Host

ing

power

are

to present

details from

number

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subsequent

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agement

self the

host. overhead

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host

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media,

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cached

2.2

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to the

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delays

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disk

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size.

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of

PBN)

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highly

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ment

cache if

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cache

disk

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configured

read/write

by knowledge

Iengt

h.

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be

accessed

disk

media.

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request,

to the far

head

of the

disk

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or seeking

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position

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priority.

Algorithms

data

data

disk

such

LBN-based

proposed

logical More

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recording,

to

aggressive

knowledge mappings

[Ruem94,

block

of the may

complexity track/cylinder

be

physical

data

unavailable

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mappings skew,

and

that

disk

often

scheduling

higher

about

First Come

a simple pohcy

results

algorithms

performance

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by

taking

requests

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ago,

[Denn6i]

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the

advantages

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ally

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request

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average

increases

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distances.

over

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disk

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in excessive

workload,

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SSTF

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that

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algorithm the

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requests

outside

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by using time

over

all

smallest

times,

variance.

to hover

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policy. by selecting

exact

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to predict

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to service

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require

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25 years

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block

layout.

achieve

Seek Delay

Over

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layout

rehes

algorithms

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information

state

shown scheduling

mm-

request.

scheduling

that

account

3.1

drive

approximations

LBN-based

have disk

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storage

cache,

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studies (FCFS)

performance.

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of the

of the

with

host

electronics.

outside

been into

starting

typically

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on-board

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use

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into

controller of the

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entities

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suboptimal

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and

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sponse

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sequential

on-board

the

data

obtained

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host

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request

offloads with

the

adapter,

total

access

process-

Interface

in terms

or no knowledge

status

overhead

to

drive,

and

media

scheduling

interface

or intermediate

approach

or controller

and

Computer

drive

associated

hand,

often

host

disk (LBN)

This

logic

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of disk

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block

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variety

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logical

the

a wide

clean

as the

the

manufacturers.

presents

the

requests.

algorithms.

on-board

a relatively

and

by

system

sufficient

protocols

(SCSI)

used

from

possess

Such

terface

read

into

Interface

disks

system.

an

10 cation

Numerous Most

of stor-

data

to service

data

ways.

read/write

satisfied

3

2.1

megabytes

to

subsequent

be determined

beyond

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charac-

of

aggressively,

switching

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these

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if the

several

request

reading

or

impact.

how

20 ms

control

buffers

cache.

necessarily

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Some

sel&contained

of increasing

lifetime is to

features

scheduling

methods

effective

of our

several

of

efforts

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and

have

2 m

disk

to prefetch

or cylinders.

disk

take

drive

prefetch

sequential

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existence

activities

cannot

rectly

might

only

containing

automatically satisfy

media

disk

speed-matching

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transfer

if the

a disk

take

embedded

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caches can

quickly

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latency

sectors

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mechanical tracks

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read/write

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drive

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is incurred

gins.

disk

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logic

to more

example, Requests

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dynamically-controlled

to

read/write

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a physical

surface,

a set of read/write

access

cylinder.

is active

each

is defined

very

difficult

is increased defect

rithm, SSTF by

time

manage-

with (for

is named

IVort94].

across

242

also

which

the

examined

provides only

the for

a marginal random

the

entire

way range

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a lower

increase

workloads the

SCAN

response

disk

or

in the studied).

arm

of cylinders,

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time

shuttles servicing

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variance

average This

than

response algorithm

back

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all

requests

forth in

its path.

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ermost

cylinder.

region

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requests

direction

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at

to middle

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changes

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more

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time

innermost

passes

both

phases

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the

center

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edges,

service.

more

variance)

than

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starvation

and

over

intervals

receive during

resists

arm

regular

cylinders

is reached

SCAN

at the

disk

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of the

effectively

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56-96

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variations

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bidirectional

seek

ing

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der

equally,

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returns

requests

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direction

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than

of

if

way.

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travel

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[Mert70]. in

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the

C-LOOK

in

[Geis87],

in

and

Table

4

The

proposed

of algorithms denotes

how

t aining alent

between

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strongly

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average

LOOK.

the

direction

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4.1

cur-

LOOK

Disk We

have to

and

algorithms,

starvation

main-

) is equiv-

rates,

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the

account

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about

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mation,

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imum

scheduler

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of data the

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request

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seek

This

aJgorithm

was denoted

provided

isting

defects.

in [Selt90]

and

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this

and

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is and

of the

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but

(SPTF)

we

use

the

to clarify

the

term

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exact

focus

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SPTF,

given

to

excessive as the requests

like

To

SSTF,

reduce

requests

that

periods request

suffers

response have

of time. ages,

are given

or

a higher

poor

variance,

been

in

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a time

from

the

priority limit

for several

(SATF)

priority

host

algorithm.

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extracted

a seek

bus

transfer

We also determined several

zoning,

of these

disks,

sparing,

simulator

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ex-

configuration,

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pending

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after

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run

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all

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arrival

workloads

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SCSI

through

with

times,

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mat ch to

delays

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activity.

the

simulator, This

varying

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and

an HP Each

delays.

in

C2247

traced

using

process

degrees times

[Ruem94]. time

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for

that

with two

square

which

re-

the

ob-

was repeated ratios,

interar-

of sequentialit

of the

horizontal

Jaco91].

ure

243

for

figure the

Figure

of our

actual

y and

lo-

and

the

disk

disk

the

measured

[Ruem94] between

run

one

can

defines

the

the

two

in

figure

time

and

validation

calibration.

shown

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response

results,

model

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cases.

difference.

for a sample

validation

distance

validation

all

this

I shows

are present.

for

in

for

by comparing

distributions

curves

a demerit

0.870

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increase

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pmtidly

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from

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starvation

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for

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simulator

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disk.

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infor-

wit h the

as Shortest

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particul.m

BN

overto model

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by monitoring

management

values

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cache

C2247.

communication

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which

rival latency).

this

and

[H P92].

set of parameters and

d-

communication

~ort94].

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media

location

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by

HP

defect

prefetch

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simula-

regions,

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physicaJ

data.

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cluding

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requested

known.

choose

(i.e.,

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blocks

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only

between

latency,

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head

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seek distances

addition,

read/write

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average

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necessary. active

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simulator

for

validated

spare

caches,

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documentation

control

recording, and control

paper, series

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and

an extensive

published

resistance.

zoned

buffers

specifications drive,

a detailed,

delays,

C2240

curve,

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bus

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ity

3.2

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heads.

thk

between

developed

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a good

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simulator

simulator

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scheduler

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tor VSCAN(R),

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cylin-

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1-4 Segments

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replaces

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along

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algorithm,

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[Seam66]. stroke

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1

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traces

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as they

[Ruem93,

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random

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systems.

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Rama92].

workloads

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of environments,

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workshift

(8 hours)

present

in

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of the

[Ruem93].

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used

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from

a version Celto

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HP

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about

vations. in which tistical machine during

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analytic packages

were

parts

distribution

environment

and

9 for

to the

line

issue rate

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is an

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factor

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incorporate

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times

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the the

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information

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tion

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agents

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VMSTM

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500

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cessing

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the

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which

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the

Repor